Two projects are described, both concerned with prospective evaluation of patients with asymptomatic cardiac conduction disease. Project one is a study of patients with intact A-V conduction and intraventricular conduction defects (bundle branch block), and is a continuation proposal for NIH contract, 71-2478. Project two is a new study, concerned with patients having electrocardiographic evidence of sinus node dysfunction (sinus bradycardia, SA block). The objective of both studies is to delineate specific patient groups with high risk of syncope and sudden cardiac death. If such groups could be delineated, significant decrease in morbidity and mortality could be expected with prophylactic demand pacing. In both projects, patients are detected by screening of inpatient and outpatient electrocardiograms. Patients are then evaluated clinically, electrocardiographically, and electrophysiologically. Electrophysiological studies include recording of His potentials, measurement of pacing responses and refractory periods, and evaluation of sinus node function utilizing atrial pacing and extra-stimulus techniques. Patients are then followed prospectively in conduction disease clinics, with specific goals of detecting significant bradyarrhythmia and sudden death. Portable tape recorder monitoring is used to increase sensitivity of bradyarrhythmia detection. It is proposed that initial evaluation will allow delineation of high risk groups (as determined by subsequent follow-up). For example, H-V prolongation might correlate with subsequent development of symptomatic trifascicular block, or prolonged sinus recovery time, with subsequent symptomatic sinus node dysfunction. All initial and follow-up data is computerized, allowing analysis with rapid information retrieval, multi- variate analysis, and life table methods.